1. Field of the Invention
The present invention relates to pumps more particularly to a plastic centrifugal pump which may be subject to running in a dry condition.
2. Description of the Prior Art
In hydrotherapy spas, tubs, pools and other receptacles, water is continuously recirculated through a motor driven pump to be introduced into the receptacle through an inlet fitting which may include an air induction arrangement in order to provide a mixed stream of water and air into the receptacle. A return or suction fitting is also provided in the receptacle in order to have a continuous flow of liquid through the receptacle.
It is standard practice in small installations such as bath tubs to empty all of the water from the tub after each use, and this also occurs from time to time in spas and pools. In this condition of the tub, all of the water will also be drained from the pump, thus leaving the pump in a dry condition. It has also become more common to utilize plastic bodied pumps in these installations and, if such a pump is run dry, the pump and its seal may become damaged due to frictional heat build up at the seal area. Normally, this seal heat is dissipated by the pumped water, however when the pump is in a dry condition, there is only limited means to conduct the heat away from the pump and seal.
Several attempts have been made to prevent damage to plastic bodied pumps which may be operated in a dry condition to prevent such damage such as including electrical circuits that disable the motor if no load is sensed, such as would occur if the pump is run dry. Other solutions have been to include electrical circuits that disable the motor if a certain level of water is not sensed in the tub or utilizing electrical circuits that disable the motor if flow is not sensed in the plumbing network.
There have also been proposed use of a metallic heat sink and radiator interposed between the mechanical pump seal and the plastic housing, thus limiting the thermal exposure of the plastic, see for example U.S. Pat. Nos. 3,826,589 and (allowed Ser. No. 865,818). Finally, timers have been proposed that would limit the run time of the pump motor which would also serve to limit damage in the event of a dry run.
All of these techniques have drawbacks either in product cost, reliability or service requirements to keep them operable in this particular environment.